1. Field of the Invention
The present invention relates to a control system for use with a continuously variable transmission which transmission incorporates a belt drive. The transmission is used to drive a generator at constant speed.
2. Description of Related Art
Aircraft electrical systems can require a power generation system which produces an electrical output at a substantially constant frequency. A method of providing this has been to use a variable ratio coupling between the generator and the engine. An example of such a xe2x80x9cconstant frequency drivexe2x80x9d can be found in U.S. Pat No. 4,609,842 wherein the constant drive takes the form of a hydraulic pump and motor and differential to achieve a constant output speed in relation to a varying input speed
GB 2220038 discloses the provision of a constant speed drive for an electrical generator in which a continuously variable transmission employing a belt is disclosed. Such belt drive technology has great potential within an aircraft electrical environment, but in order to ensure good in-service life and reliability, the clamping pressure exerted upon the belt needs to be carefully controlled. However, the belt must not be allowed to slip at any time, therefore a fast acting control system is required.
According to a first aspect of the present invention, there is provided a clamp pressure controller for controlling the clamping pressure applied to a belt of a continuously variable transmission, which transmission is in combination with a generator, the controller having a fast response mode such that in response to an input indicative of an event that may result in an increase in mechanical load transmitted through the continuously variable transmission, the controller initiates an increase in the clamp pressure in order to protect the continuously variable transmission against belt slippage.
It is thus possible to provide a control system which is xe2x80x9cprotectivexe2x80x9d of the drive belt within the continuously variable transmission. Use of a continuously variable transmission within an aircraft electrical power generation system must satisfy the conflicting requirement of high reliability and long service intervals. In use, the belt is clamped between two inclined surfaces which act to define either side of a variable ratio pulley. An increase in clamping pressure increases the rate of wear and fatigue of the belt exponentially. Thus it is desired to keep the clamping pressure as low as possible. However, should the belt slip with respect to the clamping surfaces, then both these and the belt become damaged and failure of the continuously variable transmission can result quickly. Thus it is imperative that slippage between the belt and the pulley is eliminated. This implies a higher clamping pressure which, as noted hereinabove, reduces the belt life. The clamp pressure controller of the present invention allows the clamping pressure to be optimized in order to maintain belt life, whilst simultaneously acting to rapidly protect the belt in the event of an increase in load.
Preferably the control system either monitors the output voltage of the generator, or is responsive to a device monitoring the output voltage of the generator. The output voltage from the generator may be measured either in absolute terms, or the peak values may be measured and changes in these used to infer that an increase in mechanical load through the continuously variable transmission is going to occur. The applicant has realized that mechanical inertia and electrical properties of the generator give rise to a small lag between the time at which an increase in electrical load occurs at the generator, and the time at which this increased load results in an increase in the mechanical torque that needs to be transmitted via the continuously variable transmission. This lag is small, being typically between 30 to 40 milliseconds, but can be used to prepare the continuously variable transmission such that it is protected from damage.
The control system also monitors the input speed derived from the engine speed. The input speed is used together with the generator load to determine the amount of clamp pressure.
Preferably the clamp pressure is increased by a predetermined amount to accommodate an increase in mechanical load. This increase may be a fixed increment, a percentage of the current generator load or current generator clamp pressure, an increase to a value calculated as a function of the estimated load that has occurred, or an increase to a predetermined clamp pressure, such as maximum clamp pressure. This latter option is preferred since it gives the greatest protection against slippage.
The response may vary depending on the nature of the input to the controller. Thus, some loads may be relatively low priority loads of a known magnitude. An example of this is an oven within the galley of an aircraft. A request for power by the oven may be signaled to an aircraft load controller which may then send a signal to the clamp pressure controller indicating that this known load will be switched on, either in a predetermined time period, or once the clamp pressure controller has indicated that it has prepared the continuously variable transmission for the load. Under such circumstances, the clamp pressure controller can estimate the required clamping pressure as a function of the new power demand that will occur once the oven has switched on. The clamp pressure controller can then set the clamp pressure accordingly, and once this has been achieved, it can signal to the load controller that the oven can now be switched on. However, not all loads can be conveniently scheduled in this manner, and the controller needs also to be responsive to a sudden demand placed on the generator, for example as a result of an emergency condition or failure in another generator or other device connected to the electrical system.
Advantageously the clamp pressure controller also operates in a further control mode which corresponds to a normal control mode of the controller, wherein the controller regulates the clamp pressure to obtain a minimum clamp pressure value, consistent with having a safety margin against slippage. Thus in the normal control mode, clamp pressure is regulated as a function of electrical load on the generator and the input speed.
The electrical load may be calculated as the product of the current supplied by the generator and the voltage across the generator. However, this approach may over estimate the real generator load on the continuously variable transmission since many of the load devices may be reactive, thereby giving rise to a phase change between the current wave form and the voltage wave form. Advantageously the phase change is measured, and this is used to introduce a power factor correction such that the load on the continuously variable transmission can be more accurately calculated.
According to a second aspect of the present invention, there is provided a clamp pressure controller for controlling the clamping pressure applied to a belt in a continuously variable transmission in combination with a generator, the controller being arranged to calculate a desired clamp pressure as a function of the generator output voltage, generator output current and the phase angle between the voltage and current wave forms and the input speed.